Conventionally, as described in Japanese Patent No. 4273363 corresponding to US-2009/0206827, a rotation angle detector includes a magnet rotator having four or more magnetic poles (i.e., two ore more pairs of magnetic poles), and first and second sensing devices for detecting a direction of magnetic flux generated by the magnet rotator.
Each of the first and second sensing devices is a spin-valve type giant magneto-resistance effect element having a fixed layer and a variable layer. A magnetization direction of the fixed layer is fixed to a certain direction. A magnetization direction of the variable layer is varied with a direction of a magnetic field. The giant magneto-resistance effect element has a property such that a resistance of the element is changed according to an angle between the magnetization direction of the fixed layer and the magnetization direction (i.e., the magnetic field direction) of the variable layer. When the magnet rotator rotates by an electric angle (which is an angle calculated by dividing the rotation angle with the number of pairs of the magnetic poles), the sensing device having the resistor element outputs a signal corresponding to one period of a waveform. For example, when the magnet rotator includes two pairs of magnetic poles, and the magnet rotator rotates one revolution, the sensing device outputs the signal corresponding to two periods of the waveform.
A rotation angle detector described in Japanese Patent No. 4273363 will be explained. The first sensing device includes two sensing bridges X01, Y01, each of which provides a full bridge composed of four resistor elements. The second sensing device includes two sensing bridges X02, Y02, each of which provides a full bridge composed of four resistor elements. The full bridge includes a pair of resistor elements coupled in series with each other and another pair of resistor elements coupled in series with each other. The pair of resistor elements and the other pair of resistor elements are coupled in parallel to each other between a power source and a ground. Thus, each full bridge (i.e., each sensing bridge X01, X02, Y01, Y02) is prepared. The magnetization direction of the fixed layer in the resistor element on a power source side of one pair of the resistor elements is opposite to the magnetization direction of the fixed layer in the resistor element on a power source side of the other pair of the resistor elements. The magnetization direction of the fixed layer in the resistor element on a ground side of the one pair of the resistor elements is opposite to the magnetization direction of the fixed layer in the resistor element on a ground side of the other pair of the resistor elements.
The magnetization direction of the fixed layer in the resistor elements of the sensing bridge Y01 is in parallel to a rotation direction of the magnet rotator. Further, the magnetization direction of the fixed layer in the resistor elements of the sensing bridge Y01 is perpendicular to the magnetization direction of the fixed layer in the resistor elements of the sensing bridge X01. The magnetization direction of the fixed layer in the resistor elements of the sensing bridge Y02 is in parallel to a rotation direction of the magnet rotator. Further, the magnetization direction of the fixed layer in the resistor elements of the sensing bridge Y02 is perpendicular to the magnetization direction of the fixed layer in the resistor elements of the sensing bridge X02. The magnetization direction of the fixed layer in the resistor elements of the sensing bridge X01 and the magnetization direction of the fixed layer in the resistor elements of the sensing bridge X02 are arranged to differentiate a phase by the electric angle of 90 degrees.
The first sensing device has a magnetic field sensitive direction as a reference of the magnet rotator. The rotation angle of the first sensing device with respect to the magnet rotator is defined as θ. When the sensing bridge X01 outputs the detection signal depending on a term of cos θ, the sensing bridge Y01 outputs the detection signal depending on a term of −sin θ since the magnetization direction of the fixed layer in the resistor elements of the sensing bridge Y01 is perpendicular to the magnetization direction of the fixed layer in the resistor elements of the sensing bridge X01.
Since the magnetization direction of the fixed layer in the resistor elements of the sensing bridge X01 and the magnetization direction of the fixed layer in the resistor elements of the sensing bridge X02 are arranged to differentiate the phase by the electric angle of 90 degrees, the sensing bridge X02 outputs the detection signal depending on a term of sin θ. Since the magnetization direction of the fixed layer in the resistor elements of the sensing bridge Y02 is perpendicular to the magnetization direction of the fixed layer in the resistor elements of the sensing bridge X02, the sensing bridge Y02 outputs the detection signal depending on a term of cos θ.
The factors depending on the term of θ in the detection signals of the sensing bridges X01, Y01 are defined as (X01θ, Y01θ). (X01θ, Y01θ) is equal to (cos θ, −sin θ). The factors depending on the term of θ in the detection signals of the sensing bridges X02, Y02 are defined as (X02θ, Y02θ). (X02θ, Y02θ) is equal to (sin θ, cos θ). Thus, the detection signals of the sensing bridges X01, Y02 depend on the term of cos θ. The detection signals of the sensing bridges Y01, X02 depend on the term of sin θ. Accordingly, the detection signal of the sensing bridge Y02 is reversed so that the reversed detection signal is obtained, and the factor depending on the term of θ in the reversed detection signal of the sensing bridge Y02 is defined as Y02θ′. When an operation amplifier calculates a difference of (X01θ−Y02θ′) and a difference of (X02θ−Y01θ), the value of cos θ and the value of sin θ in each detection signal are obtained. Here, a high frequency noise having the same phase is canceled in value of cos θ and the value of sin θ. Based on the value of cos θ and the value of sin θ, the value of tan θ is calculated. Then, an angle calculator executes a calculation with using an arctangent function so that the angle θ is calculated.
Here, the first sensing device is formed in a chip, which is different from the second sensing device. In this case, the rotation angle detector includes multiple chips, so that a manufacturing cost of the detector is high.
To improve the manufacturing cost, the first and second sensing devices may be formed in one chip. However, in this case, when the high frequency noise is removed from the detection signal, as described above, since the magnetization direction of the fixed layer in the resistor elements of the sensing bridge X01 and the magnetization direction of the fixed layer in the resistor elements of the sensing bridge X02 are arranged to differentiate a phase by the electric angle of 90 degrees, the electric angle increases in a case where the number of magnetic poles of the magnet rotator is small, and the dimensions of the chip increases. Here, when the number of magnetic poles of the magnet rotator is large, the electric angle is reduced, and therefore, the dimensions of the chip are limited. However, when the number of magnetic poles of the magnet rotator is large, the rotation frequency of the rotation magnetic field increases. Thus, a processing speed of the angle calculator with respect to the input signal may not be sufficient.
The magnet rotator together with the magnet is attached to and fixed to a rotation shaft. The rotation shaft is rotated by a magnetic flux, which is generated by windings. The windings surround the magnet rotator. In this case, the chip is arranged between the windings and the magnet rotator. The magnetic flux of the windings and the magnetic flux of the magnet rotator are applied to the chip. In order to detect the rotation angle of the magnet rotator based on the magnetic flux of the magnet rotator, it is necessary to remove the magnetic flux of the windings. Thus, the magnetic flux generated by the windings provides a noise, which is defined as an inductive noise.
The rotation shaft is rotated by a repulsion force between the inductive noise and the magnetic flux generated by the magnet fixed to the rotation shaft. Accordingly, the rotation direction of the inductive noise is opposite to the rotation direction of the rotation magnetic field of the magnet rotator. When the inductive noise is removed by a noise reduction method described in Japanese Patent No. 4273363, it is necessary to arrange the magnetization direction of the fixed layer in the resistor elements of the sensing bridge X01 and the magnetization direction of the fixed layer in the resistor elements of the sensing bridge X02 so as to differentiate a phase by the electric angle of 180 degrees. Thus, since the electric angle is doubled for removing the inductive noise, the dimensions of the chip much increase.